TheMo

Presentation

Studies of nanoscale objects require the development of dedicated theoretical tools : as the size decreases,
rapid variations of chemical composition and strain play an increasing role in determining the structural
and electronic properties. Continuum models (classical elasticity, envelope function theory of electronic
states) become innacurate and miss qualitative features associated with local, « atomistic » symmetries.
The use of computational approaches, modeling nanostructures at the atomistic level, becomes essential
not only for quantitative accuracy, but also for qualitative understanding of experiments. Conversely,
problematics arising from experimental activity in nanosciences sometimes question accepted ideas and
can give a strong impulse to purely theoretical work. From a theoretical point of view, nanoscience
is a new, pluridisciplinary playground where first principle methods, empirical parameter atomistic
modeling and continuum approaches closely interact.
The Theory/Modeling activity of LPN-GOSS is focused on the extended-basis spds* tight-binding model
which is particularly well suited to our objectives : this method allows to reproduce all the different
energy scales of the electronic structure (from spin splittings to full-band integrated properties like
the dielectric funtion), using universal and transferable parameters. Our work is based on a strong
interaction within a virtual team including Mikhail Nestoklon (Ioffe Institut RAS, St Petersburg),
J.-M. Jancu (FOTON, INSA Rennes) and P. Voisin (LPN-CNRS). We are developing a highly versatile
tight-binding code that can handle 3-dimensional supercells up to several hundred thousand atoms.
Topics of current interest include the modeling of STM images of subsurface acceptor states, the
electronic properties of dilute alloys (dilue nitrides, GaMnAs..) as well as methodological issues
such as the local wavefunction in the tight-binding approach.

Figure: Tight-binding calculation of exchange interaction for the neutral
acceptor state associated with a Mn dopant in GaAs.
The cross-section of impurity wavefunction in the (110) plane
containing the impurity is shown as a function of Mn spin orientation,
from [001] (left) to [1-10] (right) (M. Nestoklon et al. 2011)

Past and current Internship Training

PhDs

The extended basis spds* tight binding model is undoubtedly the best representation of single particle states in bulk and nanostructured semiconductors but from a theoretical point of view, it suffers from the lack of knowledge of the spatial properties of the basis function, which hampers the calculation of interactions between quasi-particles. In this thesis, we attempt to solve this methodological issue. In his thesis, Ramzi Benchamekh starts from a set of Slater orbitals that depend on arbitrary « screening coefficients ». He orthogonalizes them and optimizes the screening coefficients in order to reproduce the optical matrix elements between various bands, at various points in the Brillouin zone. The obtained Bloch functions are compared and yield excellent agreement with ab initio calculations.